In either scenario, bookmark originally point to Page 6 still point to Page 6 in the newly generated pdf file, which is wrong coz page 4 and 5 have been deleted, original page 6 should be new page 4 now.

Is there any way we could preserve bookmarks without doing manually shifting ? There are so many existing bookmarks, and we just need to delete two pages, it seems extra amount of work for such a simple operation.